1. Field of the Invention
This invention relates to a wireless LAN system, or in particular, to a wireless LAN system in which various communication forms such as a one-to-many communication and a one-to-one communication coexist.
2. Description of the Related Art
Types of IP communication for the wireless LAN according to IEEE802.11 and the wired LAN such as Ethernet (registered trade mark) include unicast communication, broadcast communication and multicast communication. The unicast communication is the one-to-one communication in which a base station transmits data to a single terminal address. The broadcast communication and the multicast communication, in contrast, are the one-to-many communication for transmitting data collectively from a base station to a plurality of terminal addresses. In the broadcast communication, the data are transmitted toward all arbitrary terminals existing in a predetermined communication range. In the multicast communication, on the other hand, the data are transmitted to all the terminals designated as destination.
A method of what is called a power-save mode for suppressing a power consumption of the wireless terminals is available as a standard technique according to IEEE802.11 related to the wireless LAN, as described in LAN MAN Standards Committee of the IEEE Computer Society, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, ANSI/IEEE Std 802.11, 1999 Edition, p. 128-133 (Non-Patent Document 1). According to this method, the wireless terminal switched from a awake mode to a power-save mode enters what is called a sleep mode in which it operates with low power.
Types of a sync signal (hereinafter referred to as a beacon) for the wireless terminals adapted for the power-save mode include TIM (Traffic Indication Map) and DTIM (Delivery TIM). Beacons for TIM and DTIM are transmitted periodically at a preset ratio such as 2 to 1 or 3 to 1. In the broadcast communication or the multicast communication described above, a presence or an absence of data to be received by a terminal is notified by the DTIM beacon. In the presence of the data to be received by the terminal, the data is transmitted immediately after the DTIM beacon from the base station. The wireless terminal constituting the destination address of the broadcast or multicast communication, with an arrival of the DTIM beacon receive timing, is awaken from the sleep mode and, after maintaining the awake state for a predetermined time, receives the data.
FIG. 7 shows an actual example of the broadcast/multicast (BC/MC) communication described above. In the shown case, assume that each wireless terminal (3A/3B) is in effective power-save mode. The wireless LAN base station 1, upon receipt of packets of the broadcast or multicast communication from a transmitter LAN terminal 2 (S701), accumulates the packets sequentially (S702). Also, with an arrival of the TIM beacon timing, the wireless LAN base station 1 sends out the TIM beacon (S703). The wireless terminal 3A and the wireless terminal 3B, after being awaken from the sleep mode and receiving the TIM beacon from the wireless LAN base station 1, returns to the sleep mode again.
After that, with the arrival of the DTIM beacon timing, the wireless LAN base station 1 sends out the DTIM beacon having added thereto the information indicating the accumulation of the data of the BC/MC communication (S704). Then, the particular data is transmitted to the wireless terminal 3A and the wireless terminal 3B (S705). In the meantime, each wireless terminal (3A/3B) maintains the awake mode also after receiving the DTIM beacon and receives the data by BC/MC communication.